1. Field of the Invention
The present invention generally relates to a method and apparatus for generating a training sequence code in a communication system, and more particularly, to a method and apparatus for generating and applying a training sequence code in a Global System for Mobile Communication/Enhanced Data Rates for GSM Evolution (GSM/EDGE) Radio Access Network (RAN) (referred to as GERAN) system.
2. Description of the Related Art
The 3rd Generation Partnership Project (3GPP) Technology Service Group-GERAN (TSG-GERAN) working group is concentrating on GERAN evolution to improve performance in terms of data rate and spectral efficiency. To improve downlink and uplink performance, high-order Quadrature Amplitude Modulation (QAM) schemes, 16-ary QAM (16-QAM) and 32-ary QAM (32-QAM), are added to conventional modulation schemes, Gaussian Minimum Shift Keying (GMSK) and 8-ary Phase Shift Keying (8-PSK).
A conventional symbol rate 170.833 symbols/s is added to a new symbol rate 325 symbols/s to increase data rate and spectral efficiency. The new symbol rate, which is 1.2 times higher than the conventional symbol rate, is applied to both the downlink and the uplink and is expected to be reflected in the GERAN standardization.
As described above, conventional GERAN systems adopt GMSK and 8-PSK. In GMSK, the bandwidth of binary data is limited by use of a Gaussian low pass filter and then frequency-modulated at a predetermined divide ratio. Due to the resulting continuous switching between two frequencies, GMSK boasts excellent spectral density and high spurious suppression. 8-PSK maps data to a phase-modulated code of a carrier, increasing frequency efficiency. For an EDGE/Enhanced GPRS (EGPRS) system, nine coding schemes, i.e. Modulation and Coding Scheme-1 (MCS-1) to MCS-9, are defined for Packet Data Traffic CHannels (PDTCHs). In real communications, one of various modulation and coding scheme combinations is selected. GMSK is used for MCS-1 to MCS-4, while 8-PSK is used for MCS-5 to MCS-9. An MCS is selected according to a measured channel quality.
FIG. 1 is a block diagram illustrating a downlink transmitter in a conventional GERAN system.
Referring to FIG. 1, a channel encoder 110 encodes a Radio Link Control (RLC) packet data block (referred to as RLC block) by convolutional coding and punctures the coded data in a predetermined puncturing pattern. An interleaver 120 interleaves the punctured data. For allocating the interleaved data to a physical channel, the interleaved data is provided to a multiplexer 140. RLC/Medium Access Control (MAC) header information, an Uplink State Flag (USF), and a code identifier bit 130 are also provided to the multiplexer 140. The multiplexer 140 distributes the collected data to four normal bursts and allocates each burst to a time slot of a Time Division Multiplex Access (TDMA) frame. A modulator 150 modulates the data of each burst. A training sequence rotator 160 adds a Training Sequence Code (TSC) to the modulated data and rotates the phase of the TSC. The phase-rotated data is provided to a transmitter 170. Additional components needed for transmitting the modulated signal, for example, a digital-to-analog converter will not be described in detail herein.
FIG. 2 is a block diagram illustrating a receiver in the conventional GERAN system.
Referring to FIG. 2, a radio front end 210 receives bursts in time slots through a receive antenna and provides the received data to a training sequence derotator 220 and a buffer and derotator 260. The buffer and derotator 260 buffers and phase-derotates the data. A modulation detector and channel estimator 270 detects a modulation scheme and estimates channel information using the data received from the buffer and derotator 260. The training sequence derotator 220 derotates the phase of the received data in accordance with the operation of the training sequence rotator 160 of the downlink transmitter. An equalizer 230 equalizes and demodulates the phase-derotated data based on the detected modulation scheme and the estimated channel information. A deinterleaver 240 deinterleaves the demodulated data and a channel decoder 250 decodes the deinterleaved data, thus recovering transmitted data.
FIG. 3 illustrates the structure of a normal burst in the conventional GERAN system.
Referring to FIG. 3, for data transmission, a TSC including 26 or 31 symbols is positioned at the center of the normal burst. Eight TSCs are defined in the standard, for actual use for a GSM network and a Mobile Station (MS). One TSC is allocated to each cell. In a receiver, radio channel state information is estimated from the TSC and an equalizer eliminates noise and interference from a received signal based on the channel estimation information. The receiver also measures channel quality or link quality using the TSC and feeds back the channel quality or link quality measurement to a transmitter so that the transmitter can perform Link Quality Control (LQC).
A conventional TSC is composed of codes with excellent cyclic autocorrelation properties. Hence, the conventional TSC has good characteristics when channel estimation is performed on a single channel, neglecting inter-channel interference. However, a cell is designed in a cellular system such that carrier frequencies are reused with a sufficient distance between them, considering CO-Channel Interference (CCI). As the carrier frequencies are reused more often, the CCI increases, significantly affecting the performance of channel estimation and signal detection. In this context, when the CCI is severe in a cellular system such as GSM, the use of joint channel estimation is preferred for accurate channel estimation. In this case, cross-correlation properties between TSCs have a great influence on the performance of the joint channel estimation. Yet, the present GERAN TSCs were designed with no regard to their cross-correlation characteristics. As a consequence, the TSCs degrade system performance under a CCI environment. Moreover, when the conventional TSCs are extended to high-order modulation schemes such as 16-QAM and 32-QAM used in the GERAN evolution system, they may cause the degradation of system performance.